Sage Holdings, which is backed by the Riverside Company, spent £90m on M: and D.F. King & Co, a proxy solicitation firm.
D.F.King forms the larger part of the deal, but the actual value of M: is undisclosed.
'In one sense this gives us a war chest to make strategic acquisitions,' said M: co-founder Nick Miles. He was bullish about the agency's future and said the deal confirmed his and co-founder Hugh Morrison's commitment to the firm.
Miles also said some of the funds would be used to invest in 'new and experienced talent' at the agency.
M: is currently advising Virgin on its approach for Northern Rock